In present-day communications systems, data transmitted over a high frequency channel is often subjected to degradation imparted by crosstalk. One form of crosstalk is produced by the equipment and, while it may differ from call to call, it is generally fixed for the duration of a particular call. To permit satisfactory signalling under these circumstances, a long or large multi-stage equalizer is required; however, the equalizer gain may be limited to a value which prevents the equalizer from tracking rapid variations caused by the high frequency channel. These rapid variations form a second form of crosstalk which must be compensated for an acceptable error rate in the data recovery process.
Unfortunately, conventional equalization schemes do not provide the necessary corrective action for both forms of crosstalk. Instead, basic prior art systems merely involve a conventional equalization scheme or simple cascaded schemes which are essentially further iterations of the basic equalization process. For example, the U.S. Patents to Sha et al, No. 3,708,766, and No. 3,716,807, disclose a system comprised of a plurality of identically configured equalizers connected in cascade. Successive iterations of equalizer weight adjustment procedures are carried out in an effort to reduce the distortion at the initial stage to zero. Cascaded or multiple equalization schemes are also disclosed in the U.S. Patents to Winters, No. 3,649,916 and Hirsch, No. 3,648,171. However, like the systems described in the patents to Sha et al, these systems do not carry out the dual crosstalk reduction process achieved in accordance with the present invention, summarized briefly below.